The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
A query performed on a database allows the retrieval of specific information based on the data currently in the database. Some database systems provide data to a query based on a particular “snapshot” of the database, where the snapshot corresponds to a particular time value of the logical clock used by the database server. That is, one may be able to query a database and receive results “as of” a particular logical time, thereby receiving data that reflects only those changes that were committed as of the logical time.
In one approach, implemented as part of Oracle 9i, which was released in May of 2002, a subset of all of LTV to RTV mappings are written to disk. Given that, in a busy database system, there could be many new LTVs each second, a problem with the approach is that if all or most of the LTV to RTV mappings are written to disk, then the database system would be strained. On the other hand, if the number of mappings written to disk is reduced to a number that will not cause undue processing and I/O burden on the database system, then there will be significant gaps in both logical and real time in the stored LTV to RTV mappings. For example, if an LTV to RTV mapping for the database is recorded once every five minutes, then any “as of” query could have the real timing of the data used to perform the query incorrect by as much as five minutes.
Based on the foregoing, there is a clear need for techniques that provide mappings between logical time values and real time values in a manner that overcomes the needs described herein.